1. Field of the Invention
The present invention relates to a pulse width modulation circuit for performing, for example, pulse width modulation (PWM) on an audio signal to output the modulated signal, and a switching amplifier using the same (e.g., an audio amplifier).
2. Description of the Related Art
For example, a type of a conventional switching amplifier proposed in the art uses a pulse width modulation circuit for performing pulse width modulation on an audio signal as the input signal to output the modulated signal (see, for example, Japanese Laid-Open Patent Publication No. 2004-320097). With the switching amplifier, a predetermined power source voltage is switched ON and OFF based on the modulated signal output from the pulse width modulation circuit, and the output signal switched ON and OFF is output to a load (e.g., a speaker) through a low-pass filter, for example.
FIG. 7 shows a configuration of an example of a conventional switching amplifier. The switching amplifier includes a pulse width modulation circuit 51 connected to an audio signal generation source AU, a switching circuit 52, and a low-pass filter 53. With the switching amplifier, an audio signal eS output from the audio signal generation source AU is input to the pulse width modulation circuit 51, where a pulse width modulation is performed on the amplitude of the audio signal eS. As a result, a modulated signal OUT1 and a modulated signal OUT2 in opposite phase to the modulated signal OUT1 are output to the switching circuit 52.
In the switching circuit 52, the positive and negative power source voltages +VD and −VD are alternately switched ON and OFF by switches SW-a and SW-b, respectively, based on the modulated signals OUT1 and OUT2. The output being switched ON and OFF passes through the low-pass filter 53, where a high-frequency component thereof is removed, and is supplied to a load (not shown) as an output signal V0.
FIG. 8 is a circuit diagram showing a schematic configuration of the pulse width modulation circuit 51 shown in FIG. 7.
The pulse width modulation circuit 51 is an integration-type pulse width modulation circuit using an astable multivibrator, for example, for performing a pulse width modulation on the audio signal eS as the input signal to produce and output the modulated signal OUT1, for example.
As shown in FIG. 8, the pulse width modulation circuit 51 includes a bias current source 54, a modulation circuit 55 connected to the bias current source 54, and a pulse generation circuit 56 connected to the modulation circuit 55. The bias current source 54 is for supplying a bias current to the modulation circuit 55.
The modulation circuit 55 is implemented by a so-called differential amplifier circuit, and includes resistors R51 and R52 the first ends of which are connected to each other, and transistors Q51 and Q52 connected to the second ends of the resistors R51 and R52, respectively. The modulation circuit 55 varies the current distribution ratio between first and second currents I1 and I2, which flow through the transistors Q51 and Q52, respectively, according to the audio signal eS.
The pulse generation circuit 56 is a circuit for producing a pulse signal which is to be modulated, i.e., a carrier of a PWM signal, and includes first and second charge capacitors C51 and C52, first and second inverters INV51 and INV52, first and second diodes D51 and D52, and a power source voltage 57. The pulse generation circuit 56 charges the first and second charge capacitors C51 and C52 based on the first and second currents I1 and I2 supplied from the modulation circuit 55. The pulse generation circuit 56 outputs the modulated signal OUT1 whose pulse width corresponds to the charging time of the first charge capacitor C51. Note that the predetermined power source voltage 57 is connected to the cathode side of each of the first and second diodes D51 and D52.
With the conventional pulse width modulation circuit 51, the frequency f of the carrier of a PWM signal (hereinafter referred to as the “carrier frequency”) is dependent on the bias current of the bias current source 54, the capacitances of the first and second capacitors C51 and C52, the threshold voltages Vth of the first and second inverters INV51 and INV52, etc. Therefore, where the pulse width modulation circuit 51 is used in a multi-channel switching amplifier having a plurality of channels, the carrier frequency f will vary slightly among different channels if there are variations in the capacitances of the first and second capacitors C51 and C52, etc.
As described above, with the conventional pulse width modulation circuit 51, circuits for different channels separately generate the carrier frequency f. Therefore, the carrier frequency f will vary among different channels. Where the carrier frequency f varies slightly among different channels, a beat component between signals to be modulated (carriers) is mixed in the audio frequency component, and a beat sound may be output as noise. Therefore, a sound whose sound quality is slightly altered is output from the load (speaker) as noise within the audible range.
The present invention has been made in view of the above, and it is an object of the present invention to provide a pulse modulation circuit with which the carrier frequency is made substantially constant, thus allowing for an appropriate pulse width modulation, and a switching amplifier using the same.